Human parainfluenza virus type 3 (HPF3) is an important agent of lower respiratory tract disease in children. There are currently no treatments or vaccines available to combat this pediatric pathogen. Our studies have led to understanding of crucial factors controlling virus-host cell interactions for HPF3. The overall goal of the current proposal is to expand our investigation of the molecular pathogenesis of HPF3. The proposal integrates within in its aims the contributions of the virus and the host by focusing on mechanisms whereby the hemagglutinin-neuraminidase (HN) molecule carries out its roles in the viral life cycle and in pathogenesis in the lung. Our central hypothesis is that the interaction of HN with cellular receptor is critical for several essential components of the viral life cycle - entry, fusion and release - and that this interaction regulates pathogenicity. The specific objectives of the proposal are:(1) To investigate HN-receptor interaction in HPF3 fusion and entry. (A) We will evaluate whether the quantitative relationship between HN's receptor-binding and receptor-destroying functions determines the outcome of infection. The studies are an outgrowth of our demonstration that progeny virion release depends on the balance between HN's receptor binding affinity and neuraminidase activity. (B) We will identify how HN-receptor interaction contributes to triggering the fusion protein (F) to mediate fusion. This section evolves from our demonstration that HN receptor binding is the critical first step towards HN's role in fusion promotion and that HN activates F to fuse. (2) To analyze the role of HN in pathoqenesis of HPF3 in vivo using the cotton rat model. The studies follow from our demonstration that mutations in HN that alter HN-receptor interaction (but not replication) enhance pathogenesis in the cotton rat lung. We will ask whether: (A) it is the affinity of HN-receptor interaction or HN's fusion promotion activity that is a major determinant of virulence/pathogenesis in the lung; (B) the neuraminidase of HN determines the outcome of infection in the lung as it does in cell culture; (C) 4-GU-DANA (Zanamivir), which inhibits HN-receptor binding, alters the outcome of infection in the lung as it does in cell culture; and (D) viral resistance to 4-GU-DANA in vitro translates into 4-GU-DANA resistance in vivo. The proposed studies will lend insight into important molecular events in the life cycle of HPF3 and its interaction with the host, and will assist in the design of prevention and therapy.